1. Field of the Invention
The present invention relates to a method for manufacturing a Schottky diode on a silicon carbide substrate.
2. Discussion of the Related Art
In the field of semiconductor components, the material mainly used currently is silicon. To have very high breakdown voltages, silicon carbide is preferable since silicon carbide can stand voltages per thickness unit approximately 10 times higher than silicon.
However, in the present state of the art, currently-used manufacturing processes to form silicon-based components cannot be transposed to form silicon carbide (SiC) based components. In particular, it is currently not possible, in practice, to perform implantations and diffusions of P-type dopants in N-type doped silicon carbide, noting that the P-type dopant currently used for silicon carbide is aluminum and the N-type dopant is nitrogen. Indeed, a diffusion anneal of an implantation of a P-type dopant would require temperatures on the order of 1700° C., which poses serious technological problems.
The elementary structure of a Schottky diode is illustrated in FIG. 1. This diode is formed from a heavily-doped N-type substrate 10 on which is formed an N-type epitaxial layer 11 properly doped to have the desired breakdown voltage. On this epitaxial layer N is deposited silicon oxide 12 delimiting a window in which the Schottky contact is desired to be established by means of an appropriate metallization 13. The rear surface of the component is coated with a metallization 14.
Such a structure has a very poor breakdown voltage. Indeed, the equipotential surfaces tend to curve up to rise to the surface at the periphery of the contact area, which results, especially in the curvature areas of the equipotential surfaces, in very high values of the field which limit the possible reverse breakdown voltage. To avoid this disadvantage, the structure shown in FIG. 2, in which a peripheral P-type ring 15 is formed by implantation-diffusion at the periphery of the active area of the Schottky diode, is conventionally used for silicon-based components. As a result, the equipotential surfaces must pass in volume under the P regions and thus exhibit a smaller curvature. The dopant concentration of the P-type ring must be higher than that of the epitaxial layer to sufficiently push back the equipotential surfaces in this epitaxial layer. The breakdown voltage of the diode is thereby considerably improved. As an example, with an epitaxial layer of a doping level of 1016 at./cm3, the breakdown voltage will be on the order of 10 V with no guard ring and on the order of 50 V with a guard ring.
However, as indicated previously, the forming of such a P-type guard ring is not easily implementable by implantation/diffusion in a structure formed on a silicon carbide substrate. In this case, the simple structure illustrated in FIG. 1 is not desirable either for the same reasons as in the case of a silicon substrate.